Birth-control pills, low birth weight linked

Women who get pregnant within a few weeks of taking birth-control pills seem much more likely than others to have low birth-weight or premature babies, concludes a new Canadian study that deals with one of the most widely prescribed classes of drugs.

The findings suggest couples should consider using condoms and other “barrier” contraception methods in the month before they try to conceive, say the researchers from the University of Ottawa.

The study does not definitively prove a cause-and-effect relationship and needs to be confirmed by more research, said the lead investigator. It is also unclear why oral contraceptives might affect the development of the fetus, added Xi-Kuan Chen, an epidemiologist.

The findings should not be ignored, though, he said.

“Doctors should be bringing this to the attention of patients,” said Mr. Chen, who is also a senior analyst with the Canadian Institute for Health Information. “When they consult with some patients, they should suggest there might be some effect for them.”

Babies born with a low birth weight or prematurely — a growing problem in Canada whose cause is not always clear — are more likely to suffer health problems.

One of Canada’s leading experts on threats to pregnancy, however, said he is skeptical about the team’s results, saying that most previous research has found no link between the pill and such birth problems.

Also, the study could not consider some factors that might have skewed the findings, like whether the women smoked, a leading cause of low-birth weight and prematurity, said Dr. Gideon Koren, head of the Mother-Risk program at Toronto’s Sick Kids Hospital.

It is possible, for instance, that a woman who gets pregnant soon after she was taking the pill had simply forgotten to take the drugs, indicating a careless type who might also smoke, he said.

“This clearly should not be a reason to change practices or change counselling,” said Dr. Koren of the study. “Contraceptives have been in use now for more than four decades and this issue has not come up.”

Tapping into Saskatchewan drug plan and medicare databases — among the most comprehensive in the country — the University of Ottawa researchers looked at three groups of women using oral contraceptives — those who had taken them within 30 days, 31-60 days and 61-90 days of their last period before getting pregnant –1,500 in total.

They were compared to 6,100 women who had not used birth-control pills for at least a year before they gave birth.

Outcomes were similar for most of the contraceptive users and the non-users.

Those who had taken the pill within 30 days of getting pregnant, however, were more than three times more likely to have a very low-weight newborn — under 1,500 grams (3.3 pounds) — and almost two times as likely to have a low-weight baby — under 2,500 grams (5.5 pounds). They were also twice as apt to have a baby born at least six weeks premature. The results means, for instance, that one in 42 of the oral contraceptive users had very low-weight babies, compared to one in 148 of the non-users.

The researchers say their study was unlike previous research in the area in that it isolated out women who had taken the pill very close to the time they got pregnant — where the birth effects were noticed.

But they concede the data had no information on smoking or obesity among the mothers, and that those in the 30-day group may have included a disproportionate number of unplanned pregnancies, a factor that itself might have affected the outcome.